This invention relates to centrifugal compressors for compressing a gas such as ambient air, refrigerant, or the like. More specifically, this invention relates to an improved unitary diffuser ring for use with a centrifugal compressor for providing improved compressor performance with extended operating range.
Centrifugal compressors are well-known in the art, and comprise a rotatable compressor wheel or impeller for axially receiving air or gas for compression. The impeller is rotatably driven within a compressor housing, and includes axially and radially extending compressor blades for drawing in the gas and for discharging the same radially outwardly at relatively high velocity. The high velocity gas is supplied from the wheel to a so-called diffuser section within the compressor housing which functions to slow the gas velocity thereby converting velocity energy to pressure head. The specific construction of the diffuser section may be varied widely depending upon the specific application or use of the compressor. For example, the diffuser section may be designed for a relatively high overall efficiency over a broad range of operating conditions. Alternately, the diffuser section may be designed for optimum or maximum peak efficiency for a given set of narrow flow conditions and pressure ratios, with some sacrifice in overall flow range capability.
One form of diffuser section for compressors comprises a so-called vaneless diffuser space. The vaneless space typically is formed by a generally toroidal or circumferential compressor housing which circumferentially surrounds the compressor impeller for receiving gas discharged therefrom. The vaneless diffuser space thereby forms a relatively enlarged annular volume receiving the discharged compressed gas whereby the gas velocity is reduced to yield high pressure compressed gas. Diffuser sections including vaneless diffuser spaces are advantageous in that wake effects, sonic shock effects, and the like which may be present in the high velocity and relatively unstable gas flow exiting the compressor impeller are allowed to smooth out and stabilize. This results in a relatively high overall compressor performance over an extended or broad flow range. Accordingly, compressors including vaneless diffuser spaces are desirable in applications wherein the compressor impeller may be driven through a relatively broad speed range. An example of such an application comprises a centrifugal compressor used in automotive turbochargers or the like.
Another common form of diffuser section for centrifugal compressors comprises a vaned or channeled diffuser including a diffuser ring having a plurality of generally outwardly extending radial channels for controllably receiving and expanding gas flow discharged from the compressor impeller. The ring is mounted circumferentially about the impeller, with its channels arranged for centrifugally receiving gas flow. These channels frequently are defined and separated by vanes or the line having a specific and complex leading edge configuration for helping to direct and guide the gas flow. See, for example, U.S. Pat. Nos. 1,291,037; 2,453,524; 2,967,013; 3,644,055, 3,778,186; and 3,904,312. Alternately, the diffuser channels may be defined by relatively complex combinations of passage shapes formed in the diffuser ring for handling the gas flow to meet a specific set of design criteria. See, for example, U.S. Pat. Nos. 2,311,024; 3,604,818; 3,860,360; 3,905,721; 3,964,837; and 4,027,997. In any event, these vaned or channeled diffuser configurations are typically used in relatively constant compressor speed and pressure ratio environments such as in gas turbine engines or the like.
One method of forming a vaned or channeled diffuser configuration comprises the formation of a plurality of radially angled, circumferentially arranged diffuser passages in a diffuser ring, wherein the passages are formed generally to have a circular cross section and to intersect on a common tangency circle. In order to minimize any radial separation between the tips of the impeller blades and the passages, this tangency circle is in the prior art disposed at or very near the blade tips of the impeller. See, for example, U.S. Pat. Nos. 2,708,883; 3,333,762; 3,420,435; 3,876,328; 3,856,430; 3,743,436; and 3,832,089. In this manner, any space between the diffuser ring and the blade tips is substantially minimized or eliminated whereby the compressor may optimally operate at peak efficiency at a given specific set of design conditions. Moreover, this diffuser configuration enables the diffuser ring to be advantageously formed from a single piece with the diffuser passages formed as by drilling or the like, since the step of forming the diffuser passages in the ring simultaneously defines openings extending radially through the ring for open communication with the blade tips.
Some prior art diffuser designs have been proposed seeking to combine a vaneless diffuser space with a circumferentially arranged set of outwardly expanding diffuser channels. These types of combination diffusers are advantageous in certain operating environments wherein a high peak efficiency is desired with at least some broad range operating capability, such as in refrigerant comressors for aircraft and the like. However, this type of combination diffuser design typically requires the diffuser channels to be formed on a tangency circle substantially removed radially outwardly from the diameter of the impeller blade tips in order to adequately define the vaneless diffuser space. This results in the diffuser channels failing to define openings extending radially through the diffuser ring and thereby also failing to communicate with the blade tips of the impellers. To avoid additional machining steps, prior art combination diffusers have not been formed as a single piece design, but instead have comprised multi-piece units requiring bolting or welding for assembly. See, for example, U.S. Pat. No. 4,022,541. However, these multi-piece units are not readily adapted to include circular cross section diffuser channels for optimum diffuser flow and efficiency. Moreover, the diffusers are not as commercially attractive as single piece units, nor are they readily assembled in small sizes for use with relatively small compressor impellers.
This invention comprises an improved combination type diffuser for use with a centrifugal compressor. More specifically, this invention comprises a diffuser ring including a circumferentially disposed vaneless diffuser space for receiving gas flow discharged from a compressor impeller, and for guiding the same for passage to a circumferentially arranged set of outwardly expanding diffuser channels.